Effects of SiC nanoparticles with and without Ni on the hydrogen storage properties of MgH2

In this work, MgH₂–SiC–Ni was prepared by magneto-mechanical milling in hydrogen atmosphere. Scanning electron microscope mapping images showed a homogeneous dispersion of both Ni and SiC among MgH₂ particles. Based on the differential scanning calorimetry traces, the temperature of desorption is reduced by doping MgH₂ with SiC and Ni. Hydrogen absorption/desorption behaviour of the samples was investigated by Sievert's method at 300 °C, and the results showed that both capacity and kinetics were improved by adding SiC and Ni. The hydrogen desorption kinetic investigation indicated that for pure MgH₂, the rate-determining step is surface controlled and recombination, while for the MgH₂–SiC–Ni sample it is controlled as described by the Johnson–Mehl–Avrami 3D model (JMA 3D).     

Microstructure of ball milled MgH2 powders upon hydrogen cycling: An electron microscopy study

Microstructure of MgH₂ powder particles milled for 5–60 h was studied after the powders were hydrogen cycled. Using electron microscopy we estimated individual grain size and particle size of cycled powders to compare with those before H-cycling. The nano-size grains were found to grow (3–10 times) whereas the particles undergo a mild refinement upon H-cycling. The most prominent grain growth was within the 1st dehydrogenation. The smaller grains in the milled powders demonstrated faster growth upon cycling. The average grain size estimated in this study for the cycled powders were >200 nm. The grain and particle size approached a regular or log-normal distribution with cycling. The lattice strain introduced in the milled MgH₂ powders was significantly removed by cycling for less milled powders (milled for <15 h). Longer milled (30–60 h) MgH₂ powders could not fully recover from lattice strain by H-cycles (six cycles) as observed in the present study.     

Effect of additive distribution in H2 absorption and desorption kinetics in MgH2 milled with NbH0.9 or NbF5

This paper presents a comparative study of H₂ absorption and desorption in MgH₂ milled with NbF₅ or NbH_0.9. The addition of NbF₅ or NbH_0.9 greatly improves hydriding and dehydriding kinetics. After 80 h of milling the mixture of MgH₂ with 7 mol.% of NbF₅ absorbs 60% of its hydrogen capacity at 250 °C in 30 s, whereas the mixture with 7 mol.% of NbH_0.9 takes up 48%, and MgH₂ milled without additive only absorbs 2%. At the same temperature, hydrogen desorption in the mixture with NbF₅ finishes in 10 min, whereas the mixture with NbH_0.9 only desorbs 50% of its hydrogen content, and MgH₂ without additive practically does not releases hydrogen. The kinetic improvement is attributed to NbH_0.9, a phase observed in the hydrogen cycled MgH₂ + NbF₅ and MgH₂ + NbH_0.9 materials, either hydrided or dehydrided. The better kinetic performance of the NbF₅-added material is attributed to the combination of smaller size and enhanced distribution of NbH_0.9 with more favorable microstructural characteristics. The addition of NbF₅ also produces the formation of Mg(H_xF_1-x)₂ solid solutions that limit the practically achievable hydrogen storage capacity of the material. These undesired effects are discussed.     

Catalytic effect of melt-spun Ni3FeMn alloy on hydrogen desorption properties of nanocrystalline MgH2 synthesized by mechanical alloying

To improve hydrogen desorption properties of magnesium hydride, a composite material with composition of MgH₂-5 at% Ni₃FeMn has been prepared by co-milling MgH₂ powder with Ni₃FeMn alloy either in the form of as-cast (sample A) or melt-spun ribbon (sample B). The study has shown that the addition of Ni₃FeMn alloy to magnesium hydride can yield a finer particle size after mechanical alloying (MA). As a consequence, the desorption temperature of mechanically activated MgH₂ for 30 h has decreased from 319 °C to 307 °C for sample A and to 290 °C for sample B. Furthermore, some favorable effects of Ni₃FeMn alloy on hydrogen desorption kinetics have been observed. Further improvement in the hydrogen desorption of melt-spun containing composite can be related to higher hardness value of the melt-spun powder compared to the as-cast alloy, and probably a more homogeneous distribution of the alloyed elements.     

Dehydrogenation and hydrogenation characteristics of MgH2 with transition metal fluorides

The effect of transition metal fluorides on the dehydrogenation and hydrogenation of MgH₂ has been investigated. Many of the fluorides show a considerable catalytic effect on both the dehydrogenation temperature and hydrogenation kinetics of MgH₂. Among them, NbF₅ and TiF₃ most significantly enhance the hydrogenation kinetics of MgH₂. It is suggested that hydride phases formed by the reaction between MgH₂ and these transition metal fluorides during milling and/or hydrogenation play a key role in improving the hydrogenation kinetics of MgH₂.     

Hydriding/dehydriding properties of MgH2/5 wt.% Ni coated CNFs composite

In this paper, we reported that the prepared nickel coated carbon nanofibers (NiCNFs) by electroless plating method exhibited superior catalytic effect on hydrogen absorption/desorption of magnesium (Mg). It is demonstrated that the nanocomposites of MgH₂/5 wt.% NiCNFs prepared by ball milling could absorb hydrogen very fast at low temperatures, e.g. absorb ∼6.0 wt.% hydrogen in 5 min at 473 K and ∼5.0 wt.% hydrogen in 10 min even at a temperature as low as 423 K. More importantly, the desorption of hydrogen was also significantly improved with additives of NiCNFs. Diffraction scanning calorimetry (DSC) measurement indicated that the peak desorption temperature decreased 50 K and the on-set temperature for desorption decreased 123 K. The composites also desorbed hydrogen fast, e.g. desorb 5.5 wt.% hydrogen within 20 min at 573 K. It is suggested that the new phase of Mg₂Ni, and the nano-sized dispersed distribution of Ni and carbon contributed to this significant improvement. Johnson–Mehl–Avrami (JMA) analysis illustrated that hydrogen diffusion is the rate-limiting step for hydrogen absorption/desorption.     

Influence of K2TiF6 additive on the hydrogen sorption properties of MgH2

In this study, the hydrogen storage properties of MgH₂ with the addition of K₂TiF₆ were investigated for the first time. The temperature-programmed desorption results showed that the addition of 10 wt% K₂TiF₆ to the MgH₂ exhibited a lower onset desorption temperature of 245 °C, which was a decrease of about 105 °C and 205 °C compared with the as-milled and as-received MgH₂, respectively. The dehydrogenation and rehydrogenation kinetics of 10 wt% K₂TiF₆-doped MgH₂ were also significantly improved compared to the un-doped MgH₂. The results of the Arrhenius plot showed that the activation energy for the hydrogen desorption of MgH₂ was reduced from 164 kJ/mol to 132 kJ/mol after the addition of 10 wt% K₂TiF₆. Meanwhile, the X-ray diffraction analysis showed the formation of a new phase of potassium hydride and titanium hydride together with magnesium fluoride and titanium in the doped MgH₂ after the dehydrogenation and rehydrogenation process. It is reasonable to conclude that the K₂TiF₆ additive doped with MgH₂ played a catalytic role through the formation of active species of KH, TiH₂, MgF₂ and Ti during the ball milling or heating process. It is therefore proposed that this newly developed product works as a real catalyst for improving the hydrogen sorption properties of MgH₂.     

Effects of CNTs on the hydrogen storage properties of MgH2 and MgH2-BCC composite

MgH₂ with 10 wt.% Ti_0.4Mn_0.22Cr_0.1V_0.28 alloy (termed the BCC alloy for its body centred cubic structure) and 5 wt.% carbon nanotubes (CNTs) were prepared by planetary ball milling, and its hydrogen storage properties were compared with those of the pure MgH₂ and the binary mixture of MgH₂ and the BCC alloy. The sample with CNTs showed considerable improvement in hydrogen sorption properties. Its temperature of desorption was 125 °C lower than for the pure sample and 59 °C lower than for the binary mixture. In addition, the gravimetric capacity of the ternary sample was 6 wt.% at 300 °C and 5.6 wt.% at 250 °C, and it absorbed 90% of this amount at 150 s and 516 s at 300 °C and 250 °C, respectively. It can be hypothesised from the results that the BCC alloy assists the dissociation of hydrogen molecules into hydrogen atoms and also promotes hydrogen pumping into the Mg/BCC interfaces, while the CNTs facilitate access of H-atoms into the interior of Mg grains.     

Influence of multiple oxide (Cr2O3/Nb2O5) addition on the sorption kinetics of MgH2

The influence of multiple additions of two oxides, Cr₂O₃ and Nb₂O₅, as additives on the hydrogen sorption kinetics of MgH₂ after milling was investigated. We found that the desorption kinetics of MgH₂ were improved more by multiple oxide addition than by single addition. Even for the milled MgH₂ micrometric size powders, the high hydrogen capacity with fast kinetics were achieved for the powders after addition of 0.2 mol% Cr₂O₃ + 1 mol% Nb₂O₅. For this composition, the hydride desorbed about 5 wt.% hydrogen within 20 min and absorbed about 6 wt.% in 5 min at 300 °C. Furthermore, the desorption temperature was decreased by 100 °C, compared to MgH₂ without any oxide addition, and the activation energy for the hydrogen desorption was estimated to be about 185 kJ mol⁻¹, while that for MgH₂ without oxide was about 206 kJ mol⁻¹.     

Improvement of hydrogen storage properties in MgH2 catalysed by K2NbF7

The catalytic effects of K₂NbF₇ on the hydrogen storage properties of MgH₂ have been studied for the first time. MgH₂ + 5 wt% K₂NbF₇ has reduced the onset dehydrogenation temperature to 255 °C, which is 75 °C lower than the as-milled MgH₂. For the rehydrogenation kinetic, at 150 °C, MgH₂ + 5 wt% K₂NbF₇ absorbs 4.7 wt% of hydrogen in 30 min whereas the as-milled MgH₂ only absorbs 0.7 wt% of hydrogen under similar condition. For the dehydrogenation kinetic, at 320 °C, the MgH₂ + 5 wt% K₂NbF₇ is able to release 5.2 wt% of hydrogen in 5.6 min as compared to 0.3 wt% by the as-milled MgH₂ under similar condition. Comparatively, the E_a value of MgH₂ + 5 wt% K₂NbF₇ is 96.3 kJ/mol, which is 39 kJ/mol lower compared to the as-milled MgH₂. The MgF₂, the KH and the Nb that are found after the heating process are believed to be the active species that have improved the system properties. It is concluded that the K₂NbF₇ is a good catalyst to improve the hydrogen storage properties of MgH₂.     

Effects of nano size mischmetal and its oxide on improving the hydrogen sorption behaviour of MgH2

This paper reports the catalytic effects of mischmetal (Mm) and mischmetal oxide (Mm-oxide) on improving the dehydrogenation and rehydrogenation behaviour of magnesium hydride (MgH₂). It has been found that 5 wt.% is the optimum catalyst (Mm/Mm-oxide) concentration for MgH₂. The Mm and Mm-oxide catalyzed MgH₂ exhibits hydrogen desorption at significantly lower temperature and also fast rehydrogenation kinetics compared to ball-milled MgH₂ under identical conditions of temperature and pressure. The onset desorption temperature for MgH₂ catalyzed with Mm and Mm-oxide are 323 °C and 305 °C, respectively. Whereas the onset desorption temperature for the ball-milled MgH₂ is 381 °C. Thus, there is a lowering of onset desorption temperature by 58 °C for Mm and by 76 °C for Mm-oxide. The dehydrogenation activation energy of Mm-oxide catalyzed MgH₂ is 66 kJ/mol. It is 35 kJ/mol lower than ball-milled MgH₂. Additionally, the Mm-oxide catalyzed dehydrogenated Mg exhibits faster rehydrogenation kinetics. It has been noticed that in the first 10 min, the Mm-oxide catalyzed Mg (dehydrogenated MgH₂) has absorbed up to 4.75 wt.% H₂ at 315 °C under 15 atmosphere hydrogen pressure. The activation energy determined for the rehydrogenation of Mm-oxide catalyzed Mg is ∼62 kJ/mol, whereas that for the ball-milled Mg alone is ∼91 kJ/mol. Thus, there is a decrease in absorption activation energy by ∼29 kJ/mol for the Mm-oxide catalyzed Mg. In addition, Mm-oxide is the native mixture of CeO₂ and La₂O₃ which makes the duo a better catalyst than CeO₂, which is known to be an effective catalyst for MgH₂. This takes place due to the synergistic effect of CeO₂ and La₂O₃. It can thus be said that Mm-oxide is an effective catalyst for improving the hydrogen sorption behaviour of MgH₂.     

Hydrogen absorption and desorption kinetics of MgH2 catalyzed by MoS2 and MoO2

The catalytic effect of MoS₂ and MoO₂ on the hydrogen absorption/desorption kinetics of MgH₂ has been investigated. It is shown that MoS₂ has a superior catalytic effect over MoO₂ on improving the hydrogen kinetic properties of MgH₂. DTA results indicated that the desorption temperature decreased from 662.10 K of the pure MgH₂ to 650.07 K of the MgH₂ with MoO₂ and 640.34 K of that with MoS₂. Based on the Kissinger plot, the activation energy of the hydrogen desorption process is estimated to be 101.34 ± 4.32 kJ mol⁻¹ of the MgH₂ with MoO₂ and 87.19 ± 4.48 kJ mol⁻¹ of that with MoS₂, indicating that the dehydriding process energy barrier of MgH₂ can be reduced. The enhancement of the hydriding/dehydriding kinetics of MgH₂ is attributed to the presence of MgS and Mo or MgO and Mo which catalyze the hydrogen absorption/desorption behavior of MgH₂. The detailed comparisons between MoS₂ and MoO₂ suggest that S anion has superior properties than O anion on catalyzing the hydriding/dehydriding kinetics of MgH₂.     

Optimization of TiH2 content for fast and efficient hydrogen cycling of MgH2-TiH2 nanocomposites

Magnesium hydride is extensively examined as a hydrogen store due to its high hydrogen content and low cost. However, high thermodynamic stability and sluggish kinetics hinder its practical application. To overcome this last drawback, different Ti amounts (y = 0, 0.025, 0.05, 0.1, 0.2 and 0.3) were added to magnesium to form (1-y)MgH₂+yTiH₂ nanocomposites (NC) by reactive ball milling under hydrogen gas. Thermodynamic stability of the MgH₂ phase in NCs was determined using a manometric Sieverts rig. Reversible hydrogen capacity and reaction kinetics were determined at 573 K over 20 sorption cycles under a limited reaction time of 15 min. On increasing Ti amount, reaction kinetics are enhanced both in absorption and desorption leading to a higher reversibility for hydrogen storage with the MgH₂ phase. However, titanium increases the molar weight of NCs and forms irreversible titanium hydride. The highest reversible capacity (4.9 wt% H) was obtained for the lowest here studied TiH₂ content (y = 0.025).     

Direct synthesis of MgH2 nanofibers at different hydrogen pressures

This paper describes the direct synthesis of magnesium hydride (MgH₂) nanofibers by hydriding chemical vapor deposition (HCVD), in which the effect of hydrogen pressure on the production rate, the composition and the shape of products obtained were examined by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET). The XRD patterns showed that the main product in each case was MgH₂; in particular, the products formed at 2, 3 and 4 MPa were highly pure. In contrast, at a hydrogen pressure of 1 MPa, unhydrided Mg was deposited along with MgH₂. The SEM images also revealed orientation of the as-deposited products; higher pressures of 3 and 4 MPa caused the formation of straight and curved nanofibers, and lower ones of 1 and 2 MPa, highly curved nanofibers and nanorods with a few straight nanofibers. With pressurizing hydrogen, not only the BET specific surface areas of the products but also the production rate increased. The results also appealed that HCVD could control the shape/size of MgH₂ nanofibers by changing the pressure via only a single operation.     

Synergistic catalytic effect of SrTiO3 and Ni on the hydrogen storage properties of MgH2

Study on the synergistic catalytic effect of the SrTiO₃ and Ni on the improvement of the hydrogen storage properties of the MgH₂ system has been carried out. The composites have been prepared using ball milling method and comparisons on the hydrogen storage properties of the MgH₂ – Ni and MgH₂ – SrTiO₃ composites have been presented. The MgH₂ – 10 wt% SrTiO₃ – 5 wt% Ni composite is found to has a decomposition temperature of 260 °C with a total decomposition capacity of 6 wt% of hydrogen. The composite is able to absorb 6.1 wt% of hydrogen in 1.3 min (320 °C, 27 atm of hydrogen). At 150 °C, the composite is able to absorb 2.9 wt% of hydrogen in 10 min under the pressure of 27 atm of hydrogen. The composite has successfully released 6.1 wt% of hydrogen in 13.1 min with a total dehydrogenation of 6.6 wt% of hydrogen (320 °C). The apparent activation energy, E_a, for decomposition of SrTiO₃-doped MgH₂ reduced from 109.0 kJ/mol to 98.6 kJ/mol after the addition of 5 wt% Ni. The formation of Mg₂Ni and Mg₂NiH₄ as the active species help to boost the performance of the hydrogen storage properties of the MgH₂ system. Observation of the scanning electron microscopy images suggested the catalytic role of the SrTiO₃ additive is based on the modification of composite microstructure.     

Mechanical milling of Mg,Ni and Y powder mixture and investigating the effects of produced nanostructured MgNi4Y on hydrogen desorption properties of MgH2

It has been reported that intermetallic compounds could be used to improve hydrogen storage properties of Mg-based alloys. In this study, an attempt was made to synthesize the MgNi₄Y compound from pure Ni, Mg and Y elemental powders via the combination of mechanical milling and heat treatment methods. In this regard, powders were ball milled in different conditions and then heat treated at 400 and 600 °C for 4 h to investigate the effect of temperature on the formation of MgNi₄Y intermetallic compound. The characteristics of mixtures were evaluated via (XRD) and (SEM) methods. It was found from the results of XRD analysis that ternary intermetallic compound was not formed completely via ball milling alone. Nanostructured intermetallic compound was formed after heat treatment of milled powder at 600 °C for 25 h. Furthermore, addition of 5 and 10 wt% of the produced intermetallic compound to MgH₂ decreased hydrogen desorption temperature and increased released hydrogen content.     

DFT model of hydrogen desorption from MgH2: The role of iron catalyst

Hydrogen desorption from hydride matrix is still an open field of research. Extensive, density functional theory based, ab-initio molecular dynamics simulations of MgH₂–Mg interface catches the atomic level structural mechanism leading to hydrogen desorption. The numerical model estimates the desorption temperature for an interface with Fe catalyst in excellent agreement with experimental results. Formation energies and the analysis of the structural data reveal the role played by the catalysts to lower the desorption temperature.     

Microstructural study of hydrogen desorption in 2NaBH4 + MgH2 reactive hydride composite

The desorption mechanism of as-milled 2NaBH₄ + MgH₂ was investigated by volumetric analysis, X-ray diffraction and electron microscopy. Hydrogen desorption was carried out in 0.1 bar hydrogen pressure from room temperature up to 450 °C at a heating rate of 3 °C min⁻¹. Complete dehydrogenation was achieved in two steps releasing 7.84 wt.% hydrogen. Desorption reaction in this system is kinetically restricted and limited by the growth of MgB₂ at the Mg/Na₂B₁₂H₁₂ interface where the intermediate product phases form a barrier to diffusion. During desorption, MgB₂ particles are observed to grow as plates around NaH particles.     

Site density effect of Ni particles on hydrogen desorption of MgH2

Fine Ni particles are effective in catalyzing hydrogen sorption of MgH₂, but there is confusion about the extent of this effect in relation to Ni particle size and content. Here, effects of Ni particles of different sizes on hydrogen desorption of MgH₂ were comparatively investigated. MgH₂ mixed with only 2 at% of fine Ni particles rapidly desorbs hydrogen up to 6.5 wt% around 200–340 °C, but there is no significant difference in the desorption temperature of the mixture when Ni particles vary from 90 to 200 nm. Increasing the content of Ni to 4 at%, or a combined (2 at% Ni + 2 at% Fe), leads to hydrogen desorption starting from 160 °C. Further analyses of the literature suggest that the effectiveness of Ni catalysis largely depends on its site density over MgH₂ surface, i.e., an optimal site density of catalytic particles is important in balancing the sorption properties of MgH₂. The projected trend suggests that MgH₂ can desorb hydrogen from 100 °C, the targeted temperature for fuel cells, if the number of catalyst sites is around 4 × 10¹⁴ per m² of MgH₂, or the number ratio of Ni to MgH₂ particles is about a million to one.